NCAA D1 has rules set in place to ensure that they only face D1 teams. I don't know the exact rule, but I believe as much as 70 % of any eligible D1 competition must be composed of D1 schools, otherwise coaches are forced to modify results (taking out all non D1 competition) to score the meets.
I don't really blame Oregon for not inviting non-D1 schools to compete against, since it makes there lives much easier. But, the NCAA rule set in place is really pointless and made for coaches with major ego problems. Adam's State has an incredible team this year and they deserve to race against the best. D1 is like some kind of Yacht Club or something.